Currently, photonic integration is a topic of interest because optical subsystems made from discrete optical components that are interconnected and packaged with fiber pigtails are large and expensive. Photonic integration will make it possible to drastically reduce the cost and also the size of such subsystems. This is because, based on the concept of planar parallel processing as has happened to microelectronics integrated circuits (ICs), many optical components can be fabricated at the same time. Also, since optical waveguide based photonic components are smaller as compared to free space based optical components, the component density in a photonic integrated optical module is high. Photonic integration can also enable easy short distance interconnection among the different optical components fabricated on the same substrate. This reduces the size of a subsystem as well as the packaging cost which would otherwise be needed for each optical component.
However, photonic integration also brings new challenges. For example, the beam in a standard single mode optical fiber is circular and has a diameter of about 10 micrometers. The beam of a standard III–V compound semiconductor based single mode waveguide is elliptical and has a typical size of about 1 micrometers by 4 micrometers. Beam size transformation and sub micron accuracy alignment between the fiber and the III–V semiconductor waveguide are needed when fabricating photonically integrated optical modules. Also, to take full advantage of photonic chips, which can have multiple optical inputs and outputs, simultaneous beam size transformation and the use of multiple grooves for multiple fibers are preferred.
In the past, optical beam size transformation is achieved by either fabricating a beam spot size converter directly on the III–V semiconductor material next to the single mode waveguide, and then butt-connecting an optical fiber to the beam spot size converter. Alternatively, a separate lens such as a glass ball lens, or a lens tipped fiber may be used to achieve alignment and relatively high coupling efficiency.
Improvements could be made to these approaches. III–V semiconductor based beam spot size converters are expensive, because III–V semiconductor materials such as InP are expensive (as compared to, for example, Si or SiO2). As for the ball lens or lens tipped fiber approach, light beam focusing is in air, which has a refractive index of one. The focused beam spot size cannot be smaller than the light wavelength of interest (λ=1.5 micrometers for the most popular optical fiber communication wavelength window) due to the physical light wave diffraction limit. Hence, the maximum coupling efficiency that can be achieved is only about 80%. In addition, this approach cannot be easily extended to the case where multiple input and output ports are used. In this case, each fiber needs to be aligned with each semiconductor waveguide. If one alignment fails, the whole module will not function properly.
An improvement over the above approaches is to fabricate a micro optical bench using silicon based micro-fabrication technology. However, conventional approaches using such techniques are basically designed for light beam free space connections and they do not include optical waveguide based beam size transformation couplers fabricated together with grooves and photonic chip pockets. Also, in such conventional apparatuses, active alignment is still commonly practiced, as the positioning accuracy of the photonic chips is not suitable for sub micron requirements.
Embodiments of the invention address these and other problems.